Thin layers of clear materials can reflect colors.
Thin layers of clear material can reflect light, if the thickness of the clear material matches up with the wavelength of the reflected light. A layer of nailpolish on water is as thick as a wavelength of light and so when it is captured on a black piece of cardboard a permanent oil slick is created.
To Do and Notice
Fill the tray with water.
Insert the black cardboard into the water so that it is beneath the surface. I leave one end of the cardboard sticking out of the water so that I can grab it.
Drip one drop of nail polish onto the surface of the water. Wait a few seconds until it spreads into a large circle. Lift the nail polish out of the water using the cardboard.
Notice the oil slick like colors coating the cardboard.
Allow the cardboard to dry. You now have a permanent oil slick.
What's Going On?
Light reflects off the top surface of the layer of nailpolish on the cardboard. It also reflects off the bottom surface. Depending on the thickness of the layer of nailpolish, sometimes the light from the front will be in-phase with the light from the back for one color of light leading to a strong reflection of this color. Other times the reflection from the front will be out-of-phase with the reflection from the back leading to cancellation and no reflection for one wavelength. Thus white light hitting the nailpolish layer will have some colors reflected strongly and others removed. The reflected light will be colored.
The colors depend on the thickness of the nail polish layer. The colors are a topographic map of the thickness of the nail polish layer measured in fractional wavelengths of light.
For more details about colors due to interference in reflected light look at:
Interference Model Model the reflection of light from a thin film step by step using waves drawn on paper cards to represent light waves.
Soap film colors, the complete story of colors reflected from a clear layer, a soap film.
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Scientific Explorations with Paul Doherty
18 September 2003